A mirror is an optical device, commonly made of glass,
with a smooth, polished surface that forms images by the
reflection of rays of light.
The first mirrors were likely to be a pool of water where
one could observe their reflection.
Mirrors made of brass are mentioned in the Bible, and
mirrors of bronze were in common use among the ancient
Egyptians, Greeks, and Romans. Polished silver was also
used by the Greeks and Romans to produce reflections.
Crude forms of glass mirrors were first made in Venice
in 1300. By the end of the 17th century mirrors were
made in Britain and the manufacture of mirrors developed
subsequently into an important industry in the other
European countries and in the U.S.
The original method of making glass mirrors consisted of
backing a sheet of glass with an amalgam of mercury and
tin. The surface was overlaid with sheets of
tinfoil that were rubbed down smooth and covered with
mercury. A woolen cloth was held firmly over the surface
by means of iron weights for about a day. The glass was
then inclined and the excess mercury drained away,
leaving a lustrous inner surface. The first attempt to
back the glass with a solution of silver was made by the
German chemist Justus von Liebig in 1836; various methods
have been developed since then that depend on the chemical
reduction of a silver salt to metallic silver. In the
manufacture of mirrors today, in cases where this principle
is utilized, the plate glass is cut to size, and all
blemishes are removed by polishing with rouge. The glass
is scrubbed and flushed with a reducing solution before
silver is applied, and the glass is then placed on a hollow,
cast-iron tabletop, covered with felt, and kept warm by steam.
A solution of silver nitrate is poured on the glass and left
undisturbed for about 1 hour. The silver nitrate is reduced
to a metallic silver and a lustrous deposit of silver gradually
forms. The deposit is dried, coated with shellac, and painted.
In other methods of mirror production, the silver solution
is added with a reducing agent, such as formaldehyde of
glucose. Silvering chemicals are often applied in spray
form. Special mirrors are sometimes coated with the metal
in the form of vapor obtained by vaporizing silver
electrically in a vacuum. Large mirrors are often coated
with aluminum in the same way.
In addition to their general household use, mirrors are
used in scientific apparatus, for example, as important
components in microscopes and telescopes.